One way in which laminated glass windows for vehicles have been formed in the past involves placing pairs of glass sheets to be formed onto molds that are circulated through a hot furnace for gradual heating and gravity sagging on the molds to the formed shape desired. After such sagging, the glass sheets are slowly allowed to cool to provide annealing that removes most of the stress from the glass sheets. This slow cooling is performed along a furnace section having a decreasing temperature gradient and is referred to as "active" annealing. Edge compression for such glass sheets is normally about 150 to 200 KG/cm.sup.2. Normally there is also some separation between the sheets inward from their peripheries due to thermal warpage that is subsequently overcome by laminating of the sheets to each other by the use of polyvinyl butyral in a conventional laminating process.
Laminated glass windows have also been previously manufactured by press bending which utilizes either natural convection or low level forced air cooling that advantageously produces a somewhat greater edge compression of about 300 to 400 KG/cm.sup.2. This greater edge compression advantageously makes the formed glass sheet less susceptible to edge breakage. However, since the integrated sum of the compression and tension of the glass sheet must equal zero, there is a band of tension inboard from the outer edge surface that is necessarily higher for such press formed glass sheets than for glass sheets that are actively annealed so as to have lower edge stresses. More specifically, whereas formed glass sheets that have been actively annealed normally have a net inner band tension (NIBT) of about 25 to 50 KG/cm.sup.2, press formed glass sheets typically have a net inner band tension of about 50 to 100 KG/cm.sup.2.
Formed glass sheets having a net inner band tension in excess of about 60 KG/cm.sup.2 can result in undesirable breakage which is usually tested for by the "scratch test" for windshields. In the scratch test, either the individually formed glass sheet or the laminated pair of glass sheets are abraded using 80 grit aluminum sandpaper in a band about six inches wide immediately inside the glass edge. If cracking occurs within twenty-four hours of such abrasion, the sample is considered to have failed the scratch test. This usually occurs with formed glass sheets having a net inner band tension in excess of about 60 KG/cm.sup.2 while those with a lesser net inner band tension usually pass this test. This scratch test is considered a good measure of the propensity of an installed laminated vehicle window to crack from incidental abrasion or from stone impact.
It is generally understood that edge compression of less than about 150 to 200 KG/cm.sup.2 can result in breakage of a laminated glass window during installation into a vehicle window opening, such as a front laminated windshield. Higher edge compression like the 300 to 400 KG/cm.sup.2 normally present with pressed formed glass sheets reduces the tendency of breakage during installation.
U.S. Pat. No. 4,687,501 Reese discloses lightweight bending iron heat shields for glass bending molds operable to shade glass sheets from a heat source. The heat shields are utilized to change the heating rate of the different areas of the glass sheets to affect the final shape.
U.S. Pat. No. 5,069,703 d'Iribarne et al discloses a covering for tempering of glass sheets wherein a metallic fabric is used to cover a frame that supports a glass sheet being annealed. This metallic fabric is of low thermal conductivity and reduces the heat sink characteristics of the support frame.
U.S. Pat. No. 5,285,786 Shetterly et al discloses a glass sheet annealing ring and method for reducing the maximum value of the net inner band tension while maintaining edge compression as the glass sheet is cooled. This is performed by an insulating ring juxtaposed inboard of the annealing ring to reduce the cooling rate of the glass sheet in proximity to the area where the net inner band tension is located. However, with such an insulating ring, the cooling rate is not reduced to as great an extent at localized areas where a metallic coating is provided such as at bus bars of a vehicle window heater or wiper heaters for a vehicle window.
Another problem with formed glass sheets is that current designs have relatively abrupt curvatures where breakage can take place prior to installation due to the magnitude of forces generated particularly adjacent the glass sheet edge at such abrupt curvatures. This often takes place when a pair of formed glass sheets that are laminated to each other do not have complementary mating shapes, such that stress is placed in the glass by forcing each formed glass sheet to the shape of the other.